Power-assisted steering systems are known in the art. One example type of a powered-assisted steering system is a system that includes an electrical power assist motor and that is commonly referred to as an electric assist steering system. Typically, an electrical assist steering system that utilizes a rack and pinion gear set provides assistance force by using the electric motor to either (i) apply rotary force to a steering input shaft connected to the pinion gear, or (ii) apply linear force to a steering member that has the rack teeth thereon. The electric motor in such a system is controlled in response to sensed force (e.g., torque) applied to a manually operated steering member (e.g., a vehicle steering wheel) by an operator, and possibly other sensed parameters (e.g., vehicle speed).
The torque applied by the vehicle operator to the steering wheel is sensed via a torque sensor. Torque sensors for power-assisted steering systems are known in the art. A typical torque sensor for a power-assisted steering system is operatively connected between a steering input shaft and an output shaft. The input shaft is connected to the steering wheel and the output shaft is connected to the pinion of the rack and pinion steering gear set. The torque sensor includes a torsion bar connecting the input shaft to the output shaft. The torque sensor further includes a rotary position sensor adapted to monitor the amount of relative rotation between the input shaft and the output shaft that occurs as torque is applied to the steering wheel. The amount of relative rotation is functionally related to the strength of the torsion and the amount of steering torque applied to the steering wheel.
Due to the connection between the steering wheel and the rack and pinion steering gear set that is provided by the input shaft, the torsion bar, and the output shaft, torque from the rack and pinion steering gear set may also be transmitted to the steering wheel. Typically, a certain amount of torque (e.g., a resisting force) is desirable to provide the vehicle operator a certain amount of steering feel. However, torque applied to the steering wheel as a result of some types of vehicle events may be undesirable.